At present, a large number of polymer fine particles such as nylon, polyethylene, polyacrylonitrile, polystyrene and cellulose are being used in various applications. The number of specific applications is innumerable, but examples thereof include a slipperiness-imparting agent, a toner, a matting agent for coating materials, an additive for light diffusion, an antiblocking agent for packaging materials, an insulating filler, a crystal nucleator, a packing for chromatography, an abrasive and other various additives. Furthermore, in recent years, use as a spacer for liquid crystal display devices, a standard particle for calibration of analyzers, a standard particle for assay on porous film, a carrier for diagnostic reagents, or the like is increasing.
Among these polymer fine particles, cellulose has various characteristics not shared by other synthetic polymers. As for specific examples of the characteristic, cellulose (1) is relatively chemically stable and rarely dissolvable, (2) has heat resistance and is not dissolved even at a high temperature, (3) is an amphiphilic polymer having both water absorbability and oil absorbability, (4) is derived from a natural product and regarded as harmless to human body, (5) has shapability and moldability, (6) rarely causes an interaction with a substance such as protein and causes no adsorption, (7) has many hydroxyl groups and is easy to chemically modify, (8) is easily combusted and does not generate a hazardous substance, and (9) is a biodegradable polymer and regarded as harmless to the environment.
The cellulose fine particle is adopted to a variety of applications by making use of the characteristics (1) to (9) above. The cellulose fine particle has an innumerable number of specific applications and is versatile, for example, as a column packing for various fractionations, an enzyme support, a microorganism culture carrier, a cell culture carrier, a filter element, an adsorbent, a pharmaceutical excipient, a pharmaceutical disintegrant, a pharmaceutical extender, a granulation substrate, a thickening or viscosity adjusting agent for food, a thixotropy-imparting agent, a dispersion stabilizer, a plastic extender, a filler, a base material of cosmetic foundation, a modifier for exterior coating materials, a coating agent, a molding agent for catalyst production by firing, a fiber wall material and a compounding agent for pressure-sensitive copying paper.
Cellulose is a polymer composed of a β-glucose molecule, and three hydroxyl groups present in the β-glucose molecule have a great effect on the characteristics of the polymer. When a part of these hydroxyl groups is converted into another structure, the polymer is called a cellulose derivative. The cellulose derivative has various characteristics according to the kind of the structure substituted, the substitution degree indicative of the extent of substitution, or the like. This cellulose derivative is also used in various applications by making use of its characteristics, similarly to cellulose.
In the past, the present inventors found a cellulose fine particle having both a property that the particle diameter of the fine particle is small and a property that the average polymerization degree of cellulose constituting the fine particle is sufficiently high. It was also found that surprisingly, the cellulose fine particle having a small particle diameter scarcely causes aggregation in water or various mediums even without adding a surfactant and exhibits excellent dispersion stability over a long period of time. This cellulose fine particle is a useful fine particle having the above-described characteristics of cellulose as well as a small particle diameter and may be expected to be applicable to various uses. However, as for the cellulose derivative fine particle having a small particle diameter, only a very limited fine particles are known.
The cellulose derivative fine particle having a small particle diameter, which is known at present, includes those described in Patent Document 1 and Patent Document 2. Both of these fine particles are obtained by previously derivatizing cellulose and molding it into the form of a small nanosize particle. The cellulose is basically not dissolved in water, an organic solvent or a mixture thereof but can be made soluble by derivatization, and molding into a particulate form can be achieved using a solution obtained by dissolving the derivatized cellulose. However, the cellulose derivative fine particle obtained by the method described in these patent publications is basically water-soluble and dissolves in water, and this imposes a strict limitation on the available applications. The cellulose derivative may be considered to become water-insoluble when hydroxyl groups are mostly substituted, but such a cellulose derivative fine particle cannot make use of hydrophilicity that is a characteristic of cellulose. Also in this case, the applications are very limited and furthermore, it may be feared that the fine particle itself causes aggregation in water.
In other words, a cellulose derivative fine particle that has a small particle diameter, does not dissolve even in water and can be present in a stably dispersed state, is not yet known. Such a cellulose derivative fine particle is expected to enjoy expanded usage into various applications, similarly to the cellulose fine particle. One example of these expected applications is a carrier for diagnostic reagents.
The diagnostic reagent indicates a reagent for analyzing a molecule present in the body of a living thing and detecting an abnormality or change in the body. Representative examples of the test using a diagnostic reagent include an immunoserological test, a blood test, cytoscopy and a genetic test. Also, an array used in the test for examining the sequence of an amino acid, such as a peptide array or protein array, may be included in the diagnostic reagent in a broad sense, although the test involves no analysis of a molecule in the body of a living thing. A substance differentially interacting with a substance as the test object is utilized in these tests. Among the tests using a diagnostic reagent, an immunoserological test is most representative, and this test is also called an immunoassay. The immunoassay is a test method utilizing a specific reaction between an antigen and an antibody and aims at detecting a test object substance such as cancer marker, hormone, infection, autoimmunity, plasma protein, TDM or coagulation/fibrinolysis. Such a diagnostic reagent is widely used in practice in the clinical test field because of its simplicity and promptness. Realization of high sensitivity enabling measurement of a tracer amount of a test object substance is demanded at present.
In the diagnostic reagent using a fine particle, a substance differentially interacting with a test object substance is loaded on the fine particle and a change caused when a test object substance is present is detected, thereby effecting the diagnosis. The carrier used for diagnostic reagents is generally a gold nanoparticle called gold colloid or a polystyrene nanoparticle. For example, is an immunochromatography method using a gold nanoparticle described in Patent Document 3 and a latex method using a polystyrene nanoparticle described in Patent Document 4.
However, these nanoparticles are generally hydrophobic and there are, for example, problems that the storage stability is low and fine particles aggregate and precipitate with each other, or nonspecific adsorption of causing an interaction even with a substance other than the test object substance is generated. In some cases, a stabilizer such as surfactant is used for improving the storage stability, but the stabilizer itself gives rise to the nonspecific adsorption. Also, many gold or polystyrene nanoparticles are produced using a reducing agent or an emulsifier in the production step, but such a component remains and causes nonspecific adsorption. In order to solve these various problems, a technique of covering the fine particle surface with a hydrophilic substance by using a blocking agent such as albumin is generally employed. However, the effect of this technique is not sufficient at present. Non-Patent Document 1 describes a technique of thoroughly hydrophilizing the fine particle surface, thereby suppressing nonspecific adsorption, but this technique involves laborious fine particle production and is not streamlined.
In this way, the fine particle for use in the immunoserological test is sometimes required to be a hydrophilic fine particle. Not only in the immunoserological test but also in the general diagnostic reagent field, a hydrophilic fine particle is sometimes required. Also, considering that the proportion of water occupying in the living body is very high and many of molecular reactions in the living body are performed in a water-related environment, a hydrophilic fine particle is anticipated to become useful in biomedical applications. However, the nanoparticle popular at present, such as metal, inorganic material and polymerizable polymer, is usually hydrophobic. In this context, a hydrophilic nanoparticle is demanded not only in the application as a diagnostic reagent but also in various applications.
Patent Document 1: Japanese Unexamined Patent Publication No. 2001-503101
Patent Document 2: Japanese Unexamined Patent Publication No. 2007-528436
Patent Document 3: Japanese Unexamined Patent Publication No. 10-68730
Patent Document 4: Japanese Unexamined Patent Publication No. 2000-355553
Non-Patent Document 1: Kobunshi Ronbun Shu (Collected Papers on Polymer), Vol. 50, No. 5, pp. 431-435 (May, 1993)